As the fifth most abundant element on the crust of Earth, Mg features advantages over Li such as low cost, high volumetric capacity (3833 mAh/cm3 for Mg vs. 2046 mAh/cm3 for Li) and no dendritic growth upon plating, making it a desired material for electrochemical energy storage applications. Indeed, significant research attention has been attracted to study Mg chemistries for the development of Mg batteries as a potential alternative to Li batteries. For instance, researchers have examined cathode materials that would enable facile Mg2+ intercalation for high cyclability. The divalent nature of Mg2+ nevertheless presents a critical challenge for such efforts, and only limited success has been reported. Parallel efforts have also been focused on conversion chemistries between Mg and O2. The low discharge potential and difficulty to recharge due to the spontaneous formation of MgO represent major roadblocks that must be overcome for future development toward that direction. Alternatively, the conversion between Mg and S is yet another possibility that has been explored. The low voltages (typically 0.9-1.5 V), however, significantly compromise the promises held by Mg—S batteries. Up to date, the advantages held by Mg as an energy storage material remains untapped. In response to these challenges, the present invention provides rechargeable Mg-batteries with conversion chemistry between Mg and Br2 species (FIG. 1).
Halogens have been previously explored for energy storage applications in Al—Cl2, Zn—Br2, Li—I2 and Li—Br2 systems. Compared with other halogens, Br2 offers the unique balance between energy density and chemical stability (335 mAh/gBr2; Br2/Br−=+1.07 vs SHE) and has received the most research attention. As the reactivity of Br2 would prohibit long-term utilization of aprotic electrolyte such as DMSO (dimethyl sulfoxide), THF (tetrahydrofuran) or organic carbonates, previous studies on Br2 batteries were mostly carried out in aqueous solutions. The necessity for H2O as a catholyte limits the anode choices greatly. For instance, an aqueous catholyte would prevent the utilization of Li metal as an anode unless a ceramic Li ion conductor is present, which unfortunately introduces issues such as cost, high resistivity and safety concerns due to possible leakages of the electrolyte to react with Li. Moreover, the hybrid design adopted in the Li—Br2 studies that utilize ceramic solid electrode to compartmentalize the cell components is not applicable for the Mg battery due to the lack of room-temperature Mg2+ conductive solid electrolytes. The present invention proposes a new strategy to address the challenges associated with Mg batteries.